a1 X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
a2 Department of Materials Science and Engineering and Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
a3 Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501, USA
a4 Center for Nanoscale Materials,Argonne National Laboratory, Argonne, Illinois 60439, USA
a5 Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
a6 Universidade Estadual de Campinas, 13083-859 Campinas, SP, Brazil
A series of laser pump, x-ray probe experiments show that above band gap photoexcitation can generate a large out-of-plane strain in multiferroic BiFeO3 thin films. The strain decays in a time scale that is the same as the photo-induced carriers measured in an optical transient absorption spectroscopy experiment. We attribute the strain to the piezoelectric effect due to screening of the depolarization field by laser induced carriers. A strong film thickness dependence of strain and carrier relaxation is also observed, revealing the role of the carrier transport in determining the structural and carrier dynamics in complex oxide thin films.